Carbon overcoats are commonly formed on substrates, such as magnetic thin films, in thin-film recording discs. The overcoat functions to protect the underlying magnetic layer from damage and wear caused by repeated contact between the disc and the read-write head used in accessing the disc. For this reason, the carbon overcoat is ideally formed to have a high degree of hardness or erosion-resistance.
In addition, the graphite overcoat is intended to provide lubricating surface properties, to minimize drag on the head and wear on the disc during prolonged head/disc contact. The overcoat therefore ideally provides a low-friction surface. The lubricity of a hard carbon overcoat on a disc may be enhanced by covering the overcoat with a thin liquid layer of a stable fluid lubricant, such as a perfluoropolyether lubricant. The optimum friction reduction may be achieved with a liquid layer of perfluoropolyether of about 15-30 .ANG. or higher.
A variety of methods have been used heretofore for forming carbon overcoats on a thin-film magnetic disc (Tsai). In one method, known as RF plasma or glow discharge, an RF source is used to decompose a hydrocarbon gas, producing a carbonaceous plasma whose carbon particles are deposited on a thin-film substrate to form the carbon overcoat (e.g., Natarajan; Yolamanchi; and Kobayashi). The RF discharge method is relatively slow, and deposition rates and plasma composition are somewhat difficult to control.
Another method which has been used for producing a carbon overcoat involves carbon deposition by sputtering, typically DC magnetron sputtering, in which the ionized gases are directed onto the target by magnetic fields established in the sputtering device. Typically in this method, a graphite substrate is sputtered onto a thin-layer film substrate in a low-pressure argon atmosphere until an overcoat of the desired thickness is reached.
The resulting carbon overcoat has a predominantly graphitic structure with "islands" of diamond-like crystalline clusters with dimensions on the order of about 20 .ANG.. It is, of course, the diamond-like clusters which impart the hardness properties to the overlayer. Although the overcoat formed in this manner has adequate hardness properties, it would be desirable to increase the lubricity of the layer as well, particularly lubricity after initial wear.
The need for increased lubricity is especially great in the inner diameter region of the disc, where the lubricant applied to the overcoat tends to becomes depleted over time due to migration of the liquid material under centrifugal effects, and particularly, in the inner-diameter region which is dedicated to start-stop head contact, where repeated contact with the head further depletes the liquid layer.
The above-cited co-pending patent applications disclose an improved carbon overcoat formed by carbon sputtering under an argon/hydrocarbon gas atmosphere. In accordance with one aspect of the earlier disclosed invention, it was found that sputtering voltage and pressure conditions can be adjusted to maintain hardness of the overcoat, but increase lubricity substantially over that which is achievable by carbon sputtering in the presence of argon gas alone.
It would further be desirable, for enhancing the surface wear properties of a carbon overcoat in a thin-film medium, to increase lubricant adhesion to the overcoat, to reduce loss of lubricant from the overcoat during disc operation.